This invention concerns a method for slaving a single-phase stepping motor driven by a bipolar pulse train to the load imposed by the mechanism of a timepiece. It proposes various improvements to a slaving system as described in the patent application EP 0 022 270.
The cited application sets forth a driving arrangement which permits the detection of the rotor position of a stepping motor relative to the polarity of the driving pulses and applying to the motor a pulse train of long duration if this polarity should be considered incorrect. In other words, if the rotor does not step following the application of a correct polarity motor pulse, it will receive following a predetermined time interval (one second for instance) a new pulse of the wrong polarity and it is from this moment on that the system comes into effect, the correction or recovery operating by applying to the motor two closely-spaced pulses of long duration followed by a train of pulses of greater width. None of the documents cited as prior art in the application in question describe such an arrangement.
It has however been noted that the detector as previously described presents several disadvantages which will be hereinafter set forth.
Initially, the system previously proposed in the cited application foresees two types only of pulses: narrow pulses when the torque exerted on the motor is small and wide pulses when this torque has increased beyond a certain limit. In practice, it has however been determined that such torque may take very diverse values owing for instance to one or more of the following load demands or a combination thereof: changing over of the calendar, friction in the bearings and wear thereof, ageing of the lubricants, lowering of temperature, external magnetic fields, linear or angular shocks, manufacturing tolerances, etc. In the cited application, with a choice limited to two pulse widths only, it will be necessary either to choose a first type of pulses of very short duration in order to respond to even the most minor of the foregoing load demands, or to choose a first type of pulses having a greater duration in order only to activate slaving occasionally when an important increase in torque occurs such as for a change over of the calendar. Whatever may be the chosen solution, it will be understood that the previously proposed system, although consuming less energy than a system without slaving, is not capable of reacting in a precise manner, that is to say adapting the current consumption to the real load which may be imposed on the motor.
Further, if the system of the cited application is well adapted to a stepping motor of which the poles of the stator are separated by an air gap, it is much less so adapted to a motor having saturable zones of which the poles are connected by necks of small width. FIG. 1 of the present description shows schematically a motor of which the stator poles are separated by air gaps 1. In such case, all the flux .PHI..sub.ab coming from the rotor magnet 2 passes through the core of the winding 3 in order to produce at the terminals of this winding an induced voltage Ui whenever the rotor is in motion. In the application EP 0 022 270, it has been proposed to measure the induced voltage Ui immediately following the end of a motor pulse, the winding being open-circuited. If the motor with gaps receives a correct polarity pulse, the voltage Ui measured at the terminals of its winding will be of an amplitude sufficiently high to decide whether it is possible to continue to drive the motor with small width pulses. It is however otherwise if one applies the system described in the cited application to a motor having saturable zones. FIG. 2 shows schematically such a motor where the stator poles are connected by necks 4. In this case it is to be seen that the flux created by the magnet is divided into a flux .PHI..sub.f passing through the necks and a flux .PHI..sub.ab passing through the winding core. It results from this that if one applies the system of the cited application (that is to say one measures the voltage Ui at the terminals of a winding in an open circuit) to a saturable zone type motor, one will receive an induced voltage of very small amplitude, this being unfavourable to proper operation of the control electronics.
Finally, since the cited application foresees detection of the induced voltage only after single pulses of small width where one may detect voltage of sufficient amplitude, it is not evident how to proceed in order to detect a sufficient voltage following a pulse of longer duration since as will appear from the following, the induced voltage diminishes rapidly when the control pulse is prolonged.
It is the purpose of this invention to overcome the aforementioned difficulties by proposing a method and an arrangement such as are defined in the claims hereinafter.